In at least one embodiment, the present invention is used for slush molding parts for use in the automotive industry. Slush molding is commonly utilized in the automotive industry to produce “class A” interior skins, which are used in the automobile interior, such as for instrument and door panels. A typical slush molding process involves casting a charge of powder polymeric material against a heated mold surface to form a molded part or skin with a desired shape and texture. Other processes that utilize a similar type of heated mold surface are commonly referred to as roto-molds, rotational molds and powder forming molds.
Depending upon the process and type of the part required, the polymeric material can be liquid or powder. In molding processes where powder polymeric material is used, the mold is heated to melt the polymeric material and cooled to cure the polymeric material. In processes where liquid polymeric material is used, the mold is heated to cure the polymeric material and cooled to remove the part.
The molded part may be made by attaching and sealing an open upper end of a charge box to an outer rim of an open end of the mold. The charge box is then inverted to allow the polymeric material within the charge box to fall by gravity from the charge box and onto the heated mold surface. Once polymeric material is applied to the heated mold surface, the charge box is typically returned to its upright position to allow the excess polymeric material to return to the charge box. The cast material is then allowed to melt on the heated surface. The polymeric material is allowed to solidify before removing it from the mold surface. Additionally, the mold forming surface is typically grained or textured to produce the desired skin surface texture.
A typical construction for a slush mold comprises a self-supported metal shell capable of being heated and cooled. In one particular slush molding application, the mold temperatures can reach up to 500° F. during the melting of the polymeric material. Typically, the mold temperature is controlled by using a system of heated tubes attached to the back of the mold circulating hot oil, however other methods such as hot air, infrared or induction, and heated sand can be utilized to heat the mold. Regardless of the application, the metal shell thickness is typically relatively thin with respect to the mold forming areas to permit rapid heating and cooling of the mold.
Typically, automotive slush molding tools are made of nickel. The nickel molds are typically produced using electro-deposition or vapor-deposition processes. In both cases, nickel is the desired material primarily for grain and texture reproduction, but it also produces mold shells having good corrosion resistance, wear resistance, and release characteristics.
One drawback associated with the use of nickel molds produced in this manner is that the mold can have an unpredictable, and often relatively short life cycle. For instance, nickel molds typically have a life span capable of producing between 1,000 to 80,000 parts, with between 30,000 and 45,000 parts being most typical. Since a relatively typical vehicle program has a production requirement of 400,000 to 1,000,000 parts, it is not uncommon for up to 50 molds to be needed for a program life.
The primary failure mode in nickel shell cast skin tooling is the accumulated residual stresses resulting from thermal cycling. The thermal cycle stress is a result of the material manufacturing process and the CTE (coefficient of thermal expansion) of the material which over time results in a crack in the nickel mold. Cracking can be produced by thermal expansion and contraction producing localized areas of stress due to material properties and thickness variations in the mold. This is due to the non-uniform mold thickness resulting from the manufacturing procedure. Nickel shells are typically manufactured using an electro-deposition or vapor deposition process. During these processes, the sharp points or edges of the mold will build up charge and the shell will therefore be thicker at those locations. These thicker areas will expand at different rates across the mold surface during heating and cooling and this thickness variation can produce stress over time that results in the shell failure. The shell failure is often caused by shell cracking. For instance, in applications utilizing oil to heat nickel shells, the CTE difference between the silver solder, the steel oil tubes, and the nickel shell yields three different CTE's and this mismatch between the three different materials creates stress which can lead to cracking.
As set forth above, the life of a nickel slush mold is relatively unpredictable. Because of the uncertainty of when the mold will fail, and because of the associated cost with downtime on a manufacturing line, molders of automotive parts typically maintain a relatively large number of molds on hand in order to account for and replace broken molds. In addition to the manufacturing downtime associated with replacing broken molds on a molding line and the associated costs with keeping a large number of molds on site in storage, the replacement cost of the molds are relatively expensive, as each tool costs roughly $50,000 to $500,000.
Accordingly, there is a need for a mold that has a longer life than the currently available nickel molds.